This invention relates generally to methods for spray coating the cylinder walls of a light metal engine block using a high velocity oxygen fuel (HVOF) system and more particularly the application of ferrous-based coatings.
It is known in the art to thermally spray coat the cylinder walls of aluminum engine blocks with a ferrous-based material using high velocity oxygen-fuel (HVOF) systems. Examples of prior HVOF systems include those disclosed in U.S. Pat. Nos. 5,014,916; 5,148,986; 5,275,336; 4,578,114 and 5,334,235, wherein a jet of oxygen and gaseous fuel is ignited within an HVOF gun to melt a feed wire of ferrous-based material which is expelled from the gun by the jet of burning oxygen-fuel onto the surface of the cylinder wall. The rate of application is limited by the rate of melting of the wire feed material.
It is an object of the present invention to increase the efficiency of such HVOF systems.
A method of thermally spray coating a cylinder wall of a light metal engine block includes providing high velocity oxygen-fuel (HVOF) device and advancing a feed wire of ferrous-based material into the HVOF device to locate a tip end of the wire in a high temperature zone of the HVOF device. High velocity jet flows of oxygen and gaseous fuel are supplied to the high temperature zone and combusted to generate sufficient heat to melt the tip end of the feed wire and spraying the molten feed wire material onto the cylinder wall of the engine block. According to a characterizing feature of the invention, the supply of the oxygen to the HVOF device is controlled in order to provide an oversupply of oxygen to the high temperature zone of the HVOF device in excess of the oxygen required for stoichiometric combustion of the gaseous fuel. The excess oxygen reacts with an associated fraction of the ferrous-based feed material in the high temperature zone to combust the associated fraction of the feed material as a source of solid fuel to generate a supplemental source of heat to the high temperature zone of the HVOF device.
The invention has the advantage of oversupplying oxygen to the HVOF device so as to consume a fraction of the ferrous-based feed material as a source of solid fuel so as to increase the temperature and intensity of heating in the high temperature zone, thereby substantially increasing the rate at which the ferrous-based feed material can be converted by the HVOF device as a sprayed coating on the cylinder walls. Consequently, the method of the present invention provides a more efficient process for thermally spraying ferrous-based coatings onto cylinder wall substrates in an HVOF system, increasing the application rate of the coating material and greatly increasing the number of cylinder wall surfaces that can be coated in a given time, and makes it possible to process a cylinder block using the HVOF system without use of cooling water flow in the water jacket of the block.
The invention has the further advantage of providing a simple solution for increasing the efficiency and application rate of HVOF systems with the use of standard materials, namely use of standard oxygen and gaseous fuel types and ferrous-based feed material through control of the oxygen flow relative to the gaseous fuel flow.
Still a further advantage of the invention is that the high heat capacity generated from burning the fraction of feed material decreases the dependence on the gaseous fuel as the sole source of heat for melting the feed material in the high temperature zone. The supplemental heat generated through burning of the feed material enables the user of the present invention to select from a variety of gaseous fuels, including some low cost fuels which, on their own, may not provide sufficient heat in an HVOF system for acceptable performance of the system. However, supplemented by the burning of the feed material as a solid fuel source, these otherwise inadequate gaseous fuel sources become viable as low cost alternatives in an HVOF system as the gaseous fuel source.
A further advantage of the invention is that the burning of a fraction of the ferrous-based feed material produces iron oxides which are incorporated as part of the thermally sprayed coating. The presence of iron oxide particles increases the wear resistance of the thermally sprayed coating.
According to a further aspect of the invention, aluminum may be added to the ferrous-based feed material to lower the oxygen content in the sprayed coating and to alter the form of oxide from FeO to FeAl2O4. FeO is a metastable oxide phase that can transform over time at engine operating temperatures to Fe3O4 in a volume expanding reaction. FeAl2O4 is a stable oxide phase that is not subject to any transformations at engine operating temperatures. The presence of the aluminum in the oxide further enhances the wear resistance properties of the thermally sprayed coating and is less brittle than a coating having FeO oxides.
According to a still further aspect of the invention, additives are included in the iron-based feed material to control embrittlement from impurities such as sulfur. According to the invention, introducing yttrium, calcium, magnesium, titanium, zirconium, hafnium, cerium, or lanthanum has the beneficial effect of tying up impurities so as to eliminate their ability to segregate to interfaces such as grain boundaries to reduce or eliminate embrittlement caused by such impurities.